1. Field of the Invention
The present invention relates to an optical apparatus for effectively controlling the quantity of light for observation in response to the variations of image magnification in an imaging system and particularly to an optical apparatus conveniently usable for observing the mask and wafer in an aligner.
2. Description of the Prior Art
To observe an object, there are generally used two methods, a method in which the object is visually observed through an eyepiece as in the conventional microscopes, and another method in which the eyepiece is replaced by a TV camera through which the image of the object is formed on a TV monitor and indirectly observed. The latter method is inferior to the former method in its resolving power and dynamic range for variations of light quantity, but has been currently utilized because of many advantages such as easy observation, safety and others.
In an optical system having its variable magnification such as a zoom erector lens incorporated into the microscope system, the change of magnification causes the F-number of the beam at the image observation side to vary correspondingly. The F-number at the image formation side varies in proportion to the variations of the image magnification. For example, if the image magnification is changed from .beta..sub.1 to .beta..sub.2 (.vertline..beta..sub.2 .vertline.&gt;.vertline..beta..sub.1 .vertline.), the quantity of light received by the image pick-up surface of the TV camera will decrease to (.beta..sub.1 /.beta..sub.2).sup.2. In other words, if a ratio of zoom is .beta..sub.2 /.beta..sub.1, the ratio of light quantity is more greatly changed by the square of the inverse number of the zoom ratio. If the range of sensitivity in the image pick-up tube or solid-state image pick-up element does not follow the change of light quantity caused by the change of image magnification, the quantity of light is too much decreased at the telescope side (long focus) or saturated at the wide-angle side (short focus) so that the image cannot be resolved. Furthermore, it is hard to visually observe the object.
If there is used a source of illuminating light having the quantity of emitted light determined by the magnitude of a current for energizing said source, such as a halogen lamp, the light emitted from the light source can be increased when the light incident on the image pick-up surface is not enough, and on the contrary decreased which it is too large in quantity. However, such a system results in complicated structure since it requires a photoelectric element for measuring the quantity of light incident on the image pick-up surface and also a servo mechanism for determining the current to be supplied to the light source depending on the output of the photoelectric element. Particularly, the halogen lamp sometimes has to be used beyond its rating if the quantity of light is needed to remarkably change. This will adversely affect the life and halogen cycle of the lamp.
If the source of light is in the form of a laser, the output of which is hard to be changed, the above-mentioned control does not work.
It may be considered that the quantity of light is controlled by changing the diameter of an aperture stop as in the usual zoom lenses. However, the aperture control cannot be applied to the microscope system because the numerical aperture is decreased, resulting in poor resolving power, if the aperture is reduced.